A scaler, or pixel/line sample rate converter, is a device that can resample a video horizontally or vertically. A scaler is an essential requirement for a digital video processor, in order to fit the incoming video to the desired display resolutions.
The ratio of input resolution to the desired output resolution is called the scaling ratio, which generates a linear mapping of the output pixel positions of the target video frame to the input pixel positions in the source video frame. Depending upon the mapping, a filter corresponding to the mapped position is picked from a bank of poly-phase filters, for generating each output pixel. Using appropriate filters during scaling for various scaling ratios ensures a clean, non-aliased and sharp output picture.
Since good quality vertical scaling operation requires more than 1 tap, scaler hardware needs to have access to either line buffers storing a few lines or a frame buffer. A set of dedicated line buffers is more economical than a frame buffer, since this also saves or decreases use of SDRAM bandwidth by a scaler when there are multiple applications accessing the SDRAM. However, there may be an associated area disadvantage of having a separate set of line buffers exclusively for a scaler. Accordingly, the designs for a particular scaler may vary greatly depending on the situations and applications to which the scaler is applied.